Many different gear systems are known in the prior art. One such system is a simple epicyclic gear train or planetary gear system. The planetary gear system consists of a sun gear, 1 or more planetary gears, a ring or annulus gear and a carrier for the planet gears. The number of planet gears varies considerably in actual practice; with some ratios it has been possible to utilize as many as twenty planet gears, while sometimes as few as one is used for light load applications. Planetary gear systems are widely used because they are compact, strong and have no non-linearities between their input and outputs. Their drawbacks are that they do not provide high torque amplification and are not anti-backlash.
Another type of prior art gear system is the harmonic drive. Harmonic drives consist of a wave generator, a flex spline and a circular spline, and are either cup type or pancake type drives. When the wave generator is rotated, it presses against the flex spline in two diametrically opposite points and distorts the flex spline in the process. This distortions force the gears on the outside of the flex spline to push on their opposite number gears on the inside of the circular spline. This, in turn, causes the two splines to rotate with respect to each other. This mechanism is essentially frictional, but with the motion from the inside out, the engagement angles are very steep and efficiency and torque multiplication is very good, especially for the cup type. Harmonic drives are, however, subject to ratchetting under large loads as they only engage two gear teeth at a time. The flex spline can deform under load which makes it easier for the teeth to yield. The flex spline is also prone to overstressing, and lubrication can be a concern. Harmonic drives have nonlinear harmonic distortions in the relationship between their, input and output motions which complicates control factors. They are nearly anti-backlash, although they do have a small amount. The pancake version, although compact, has low efficiency and lubrication problems, while the cup type, though efficient and easier to lubricate, has long axial lengths and thus not compact.
Anti-backlash gears are commonly configured as two identical gears joined together by a torsion spring. These gear halves counter-rotate until each seats on the opposite face of the respective gear teeth it is fitted to. For very light loads, this acts as an anti-backlash system. For larger loads, however, the spring is easily overpowered and a situation exists where one of the gears takes the full force of the load first.